(1) Field of the Invention
This invention relates to the field of photocomposition using electronically displayed character images generated from stored binary signals.
(2) Discussion of the Prior Art
Electronic graphic displays controlled by digital computers are presently used in a variety of applications including computer aided design, long distance telecommunications and word processor systems. Due to their extremely high speed and great versatility, computer controlled CRT displays have even found application in the field of photocomposition of typecharacters and other graphic symbols but such applications have generally been attended by low resolution and high cost due to the vast amount of digital data needed to obtain even a minimally acceptable character resolution. As the requirement for greater resolution in character design increases, very significant sacrifices must be made in the speed at which the character designs are displayed in order to keep the capital equipment costs within reasonable limits. For this reason, virtually all photocomposition systems capable of producing high resolution, graphic quality character images have relied upon film grid fonts from which the character designs may be optically reproduced. Film font systems, however, suffer from a number of disadvantages including the high cost and fragile nature of the film grids, the need for a complicated font, support mechanism and the need for an expensive optical projection system.
Since an electronic display system virtually eliminates all of these disadvantages of the conventional film font, numerous attempts have been made to develop a practical electronic system capable of producing sufficiently high resolution to compete with film font based photocomposition systems. In U.S. Pat. No. 3,569,951 to Lavenir, a digital computer based graphic symbol display system is disclosed in which line image characters are generated on a CRT display screen by cursively moving the CRT beam in response to a series of 3 bit codes commanding successive translational movements of the CRT beam. Since the CRT screen can be imagined as an orthogonal matrix of dots, each translational movement of the CRT can be described as a movement from one dot in the matrix to an adjacent dot in one of eight possible directions (called Freeman directions). A three bit binary number is required to identify all 8 possible directions assigned to each translational movement command produced by the digital circuitry controlling the CRT display. To obtain a greater degree of flexibility, the Freeman direction codes can be expanded to allow selectively for either one dot or two dot translational movements, as is disclosed in U.S. Pat. No. 3,533,096 to Bouchard and U.S. Pat. No. 3,603,967 Houerbach.
Still further reduction in the storage capacity required for cursive character generation can be realized by using successive two part encoded commands wherein the first part of each command identifies generally a sector direction in which movement of the CRT beam will take place and a second part specifically identifies a path within the sector over which the CRT beam is to be moved. By generating successive two part commands of this type the CRT beam may be commanded to sweep out any arbitrary design. Examples of this technique are disclosed in U.S. Pat. Nos. 3,675,230 to Pitteway; 3,716,705 to Newell and 3,735,389 to Tarczy-Hornach. While significant reduction in storage capacity can be achieved by this approach especially when a large number of display matrix dots are traversed in response to each binary path identification code, this reduction is offset somewhat by the need to include a number of bit positions in each command to identify the direction sector in which path movement is to take place. Moreover, cursive line generation of character images allows no variation in the thickness of the line images generated and is therefore unacceptable in most situations in which graphic quality photocomposition is desired.
Accordingly, it has been suggested to encode additional information such as disclosed in U.S. Pat. No. 4,087,788 to Johannesson in which the Freeman direction codes are supplemented by digital information relating to the "thickness" of the various letter portions. Some loss of resolution occurs in systems of this type and thus for very high resolution work, the system disclosed in U.S. Pat. No. 3,581,302 to Kolb may be employed wherein successive 3 bit Freeman codes are employed to describe in successive translational movements the position of all dots in the dot matrix of a CRT display which must be illuminated in order to recreate a particulur character image. The Kolb patent recognizes that one of the Freeman directions may even be eliminated by careful arrangement of the instructions and yet permit all of the dot positions to be described. In this way the eliminated Freeman direction code can be used for further machine code instructions without requiring more than three bits per translational command code.
A system using successive 3 bit Freeman codes to define each dot location of a character design will maximize the resolving capability of a CRT display, but massive storage capacity will be required to approach the maximum resolving capability of the human eye. For example, assuming the minimum resolving capacity of the unaided human eye at a normal reading distance to be about 0.0002 inch, a character reproduced at copy size in 12 point type would require almost 1 million dots of 0.0001 inch to define a dot matrix covering a 12 point EM square which is the imaginary square in which all letters of a 12 point alphabet are formed. Even if the letter form uses only one-tenth of the dots in the EM matrix, 300,000 bits of storage capacity would be required for each symbol in the alphabet in order to achieve the maximum visibly perceptible resolution in the output image.
One technique for reducing this mammoth storage requirement is illustrated in U.S. Pat. No. 3,594,759 to Smura wherein successive 24 bit computer commands are sent to a decoder circuit for deflecting a CRT beam in a pattern to sweep through the dots defining one portion of a character. Basically the system of the type illustrated in the Smura patent works well for "block style" lettering, but tends to break down when the letter boundary is of a curvilinear, nature. Note for example the chart in column 8 wherein 30 24 bit commands are required to describe a curved letter portion as compared with rectangular portions requiring only 2 or 3 24 bit commands.
An alternative approach to encoding commands identifying all dots making up a character is to encode only the boundary point of the character design and to use these encoded boundary point positions to control a raster scanned display to recreate the character image. A system of this type is disclosed in U.S. Pat. No. 3,783,331 to Darnall wherein original artwork is scanned in raster fashion to produce signals indicating the position at which each scan line crosses the boundary of the character. This stored information is subsequently read out to control the blanking and unblanking of a CRT beam which is raster scanned over a display screen to recreate an image of the character. In a system employing many hundreds of scans per character, the amount of storage capacity required can still be impractical with this system even though significant advantages are achieved over systems identifying the location of each and every point in a character image. Moreover, a system of scanning original artwork such as illustrated in U.S. Pat. No. 3,783,331 requires simultaneous scanning of the artwork and a reference character in order to obtain a spatial reference for the encoding data. This requirement prevents the selection of the conventional base and left hand reference lines normally used by typeface designers as the scan reference since the character design will often touch the conventional base or left hand reference lines thereby creating the absence of a reference character in the area of overlap. Other techniques for creating character images by raster type scanning of a display screen are illustrated in U.S. Pat. Nos. 3,422,737 to Bailey, Jr.; 3,643,067 to Coldita et al and 3,713,098 to Muenchhousen et al.
Some attempts have been made to combine the benefits of cursive type character data storage with the efficiency and simplicity of a raster scanned image display. For example U.S. Pat. No. 3,936,664 to Sato discloses a technique whereby a character pattern is encoded by end to end vectors defining plural dot positions whereby the stored vector signals are used, upon decoding, to store data bits in a random access memory in which the storage cells correspond to dot positions in an electronic display matrix. When all of the vectors making up a character have been stored, the memory is read out to control a conventionally scanned CRT display.
Another way to combine cursive type character encoding with raster scanned output display is illustrated in U.S. Pat. No. 3,870,922 to Schutoh which discloses a pattern generating structure wherein the coordinates of boundary points of a pattern intersected by a scan line are generated in real time using encoded data relating to successive translational movements from one boundary point to another. The CRT beam is unblanked when the position of the CRT beam coincides with a coordinate being generated from the encoded data indicating that the beam is entering the pattern image and is blanked when the CRT beam position coincides with a coordinate being generated from the encoded data indicating that the beam is leaving the pattern image.
Attempts have also been made to achieve greater data compaction by modifying the organization of the storage media itself. For example, numerous techniques have been developed, such as illustrated in U.S. Pat. No. 4,001,883, for high density data storage on magnetic discs using uniform length data sectors. Similar techniques such as disclosed in U.S. Pat. No. 3,514,616 to Kolb have been disclosed as being particularly advantageous for the storage of encoded data CRT image generation wherein the data for each character is subdivided into subsections assigned to plural sectors containing both character and non-character data. Disc storage media organized with uniform length data sectors inevitably result in unused storage capacity since the amount of encoded data necessary to describe completely any one character will be variable and will often require only a fraction of the last data sector assigned to record the encoded character data.